Do you know if your friend is good at their job? How do you know this?
Think carefully, as you have two options:
1) Your friend asks personally, leaving you out of the loop
2) You ask the client. Whether you use the words or not, you are sating to the client “I vouch for this person, and their ability to do the job in a professional manner”. If anything goes wrong, then the client will hold you responsible to some extent, and your relationship will change, and it is unlikely to be for the better. In the worst case, the client may complain to your boss, or may even withdraw their custom from your firm.
I know that the latter is probably unlikely, but that’s doesn’t mean that it is impossible.
I have recommended only a few friends in a career of "several" decades.
Generally, I will just tell those who ask the name of the hiring manager and request them not to mention me. So far, no one has been upset about this. – they go the lead after all.
The few whom I have recommended are people with whom I have worked directly, and in who’s technical skills and personality I have faith.
The client may or may not have an opening. It is unlikely that he will create one for you. So, why can’t your friend ask directly?
If you ask, the potential upside is that your fiend may learn about an opening hat the could have learned of themselves. The potential downside is probably something that you would prefer to avoid.