If he is familiar with bikes than he should go with her. He will know what he will ride (or won't) and what he wants (or doesn't want). It ruins the surprise, but it is the thought that counts.

Let him know that the budget is a certain amount and let him look around. It will be like a kid in a toy store. Plus it will be a process that they can enjoy together. Watching him run from one bike to another pointing, and saying "I want, I want" with a big grin on his face. Eagerly asking the salesman to pull down the bike so he can take it on a spin. Watching him ride in the parking lot going "weee", and "look at me, no hands". His wife yelling at him "stop that, your gonna break it".

Those moments are priceless. (Uh, how old is he again) ;-)