If he isn't turning up all the way he will be fine.
There is nothing wrong with running two cabs that are rated at different wattages as long as you don't exceed what the lesser watted speaker can handle.
This thread is getting a little long in the tooth, and is probably well settled by now, but I feel i owe Joe a response.
What you've said here Joe is very true. The gotcha I see is that none of us, except for the OP, knows how loud the guy plays. If he is in a loud band then his 15 will be in constant danger. We also have to remember that most power ratings are for the thermal limits of the drivers. Over excursion is really bad for any driver and in general that happens well before the thermal limits.
Paul
Thanks guys!