Depends on if the OP considers that a fair trade. Doing a 10 minute work task when you're already at work can be quite different from having to rearrange your weekend schedule to make sure you have your laptop with you and some place where you can sit down to work for 10 minutes at a specific time of the day. The OP doesn't say what their original weekend plans were, but it could be a major blocker if they'd planned to e.g. see a movie or go to a concert, and awkward if they were going to be traveling or hanging out with friends or meeting relatives or going somewhere with their kids, etc.

Option 2 sounds very passive-aggressive to me. You can "make it clear that you don't begrudge having already covered for him" as much as you want — the fact that you still remember the issue at all the next week clearly tells your coworker that you do begrudge it, and if they're not a totally insensitive asshole, they'll feel bad that you waited to bring up the issue until it's too late for them to do anything about it except feel guilty.

+1 for "Are you sure things have not already escalated to this level?"

(OK, maybe you can know that, if there's an established process that you know well enough to predict exactly what they'll do, or if you're involved enough in the process to keep dropping veiled hints that it's a bad idea right now or something. But it still seems like a very risky approach: you're basically trying to get your company to do something involving the client, but without telling the client, and without telling your company why they shouldn't tell the client.)

But if you don't tell your colleagues involved in the process about the fact that the client talked to you in confidence, how can you be sure that they won't go ahead and give the client a call?

@DKNguyen: The OP's personal interests in siding with their employer should be obvious: being seen as a good employee contributes to continued employment and may earn the OP performance bonuses or maybe even a promotion. Their reasons for siding with the client, besides emotional attachment and maybe a feeling that they "ought to" do it since the client trusts them, are less clear. But they might have some, e.g. if it's plausible that they might end up directly employed by the client in the future. (That happens more often in these kinds of close client relationships than you might think.)

@GregoryCurrie: The client's and the employer's interests clearly conflict here: the client does not want the OP's employer to know something that it would be in the employer's interest to know. The OP is caught in the middle here and, reasonably or not, seems to want to please both parties, which they cannot do: telling their employer is against the client's interests, while not telling is against the employer's interests. Hence, conflict.

… Also, if the OP does as you suggest at the end, and their employer ends up calling the client with a retention offer just after the client "confidentially" told the OP they were thinking about leaving, the client will put two and two together and assume the OP broke their confidence. And while the OP's colleagues could perhaps handle the situation in a way that makes it seem more like just a plausible coincidence, they won't know that they should do that because the OP never told them.

This answer seems to be basically predicated on the assumption that the OP's bosses are idiots and don't understand the concept of "possibility" as described in your answer. If that is indeed the case, and they're as prone to take any random rumor they hear as gospel as you seem to assume, then yes, the OP probably shouldn't tell them anything that isn't 100% certain to be true. But if the OP's bosses are that stupid, the OP should probably also consider finding new employment elsewhere.

@GregoryCurrie: It's hard to imagine a scenario where "the best interests of your employer" would truly be served by hiding information from your superiors in the company, at least assuming that said superiors are themselves even halfway competent. (Obviously your client might not like it if they knew you disclosed their secrets to your employer, but if you're going to betray someone's trust anyway, you might as well not tell the client.) Your employer can then decide whether to act on the information (and thus potentially reveal to the client that they know it) or not.

To say the same thing as @Joe in slightly more words, that only works if the majority of the employees feel that "getting answers from you to important business questions" is more important than "making fun of you and your silly town hall." (And it might actually take a significant supermajority due to vote splitting.) As a newcomer trying to institute a new collaborative practice while dealing with both change resistance, general cynicism towards management initiatives and what appears to be significant hostility inherited from your predecessor, I wouldn't count on having that.

+1, but do make sure that the hotel you suggest is one you're actually willing to stay at more than once, because if your boss approves it, it also sets a baseline for what you're willing to consider "reasonable accommodation" on future business trips to that city.