Safety pilot time is not the greatest way to build commercial time, but I suppose it is technically legal. If the OTHER guy asked the question I would tell him not to use safety pilot time towards HIS commercial.

But it sounds like you'll be the hood pilot, not the safety pilot? If so, I suppose there's nothing wrong with hood time towards commercial (as long as your required VFR commercial X-countries are not done under the hood). Doesn't sound like too bad a deal to me, but get it in writing from the school or FBO...if that CFI gets an airline job, who's going to provide your free training?