Startups are crazy. I know, I've worked for several and currently have my own. These are not places where the "normal rules" apply. Meaning, if Sally takes a week off then likely anything Sally was working on won't be picked up by someone else - there is no "fat" here, everyone is required. Point is that if there is a looming deadline and a member of the team takes a holiday, then this can have very real consequences for everyone else. That vacation notice given three months ago probably seemed so far off that the owner didn't give it a second thought. But now it's here and causing a problem.
Startups are crazy. One day everything looks peachy, then just one day later everything is burning. I've seen this type of roller coaster ride happen several times in a single day. This makes for a very stressful environment that not very many people can handle.
Startups are crazy. Let me rephrase that: you have to be crazy to start a business. It's hard, really really hard. On the one hand the founder(s) have a vision, on the other reality intrudes. This "reality" is that startups often have to deal with tremendous scope creep just to sign a deal. At some point you'll have enough real paying clients to be able to "just say no" to that newly demanded feature. It doesn't sound like this particular business is there yet. Half of our primary product was built for these reasons and many other companies have completely changed direction based on what they could actually sell.
Startups are crazy. There are times where the bank account approaches (or worse, passes) $0. When this happens the founders have to make a conscious decision to either give up or push through. Whatever happens the founder(s) #1 job is to ensure that everyone is paid. I've been there. I've put my own salary on hold for months just to ensure that my people are paid. I've laid off good strong employees because I knew I could pay them now but in 30 days the likelihood of having enough cash was really low. I've begged clients to pay invoices a month or two early and I've borrowed money from family just to make payroll.
It would be trivial to say this is the founders problem. However it's actually everyone's. The entire staff has to want to see this succeed. Which means showing up to work and doing everything you can to make it happen... including changing plans.
Startups are crazy. Because of the emotional joy ride they can be both incredibly fulfilling and the absolute worst experience of your life. Even at the same time; and few people are willing to subject themselves to this. I like to call those people "sane", but as a founder I'm probably not qualified to judge.
My advice: If the employee isn't comfortable working in an environment like this then they need to find their exit as quickly as possible. I'd suggest they cancel the trip no matter what and focus on the day job while locating a new place to work.
If the employee is happy in this environment then I'd suggest they cancel the trip and buckle down to do everything they can to make this deal work.
Is it right or even fair? No, not really. However "right" and "fair" have little to do with trying to get a new company off the ground. The question she needs to ask herself is "how far is she willing to go to see it succeed?"