It happens, every once in a RARE while, that someone brings an older child to one of our playgroup's events, and then only if it's really necessary. The mom generally asks if it's okay, and we do appreciate the heads-up. Is it possible your friend mentioned it in this sort of context? She's told you ahead of time, so that makes me think she's trying to feel out your opinion, anyway.
We do have evening and weekend events from time-to-time, and dads and older siblings are always welcome to these, but during regular playdates, the older kids are generally painfully bored. Definately point that out - that it would be a very bad idea for him to come to in-home playdates, as there would be nothing for him to do, since most of the houses you'll be playing in don't have older siblings, and no age-appropriate activities for him. What will he do to entertain himself? I would be uncomfortable, having a bored teen just wandering around MY house...(The occasional heads-up is one thing, and can be arranged for - but it seems a bit much to expect hostesses to entertain a teen for weekly playdates.)
However, if you're going to the park, maybe suggest he bring (or meet) a friend of his own, and a soccer or basketball, and have his own sort of mini-"play"date on the side?