Serveral reasons why I think this is a bad idea.
1. How well is your boss going to take instruction from you? Are you going to be able to critque him honestly from a subordinate position? How secure do you feel in your job if he is not happy with his progress?
2. Do you work as a corporate pilot now? If so aren't you essentially instructing yourself out of work?
3. If you are the company's pilot now, you are giving the boss ammunition to criticise every operation decision he doesn't like. A little knowledge...
4. If he is like many bosses, he will want to combine work trips with instruction. While some of this can be productive, it gets to be a waste after a while.
5. The financials. Who is going to pay for the additional insurance, both for the aircraft and you? Who will be responsible for the 172?
6. I tried this once. It was terrible. The guy never even soloed. Ground kept getting put off for meetings. "Gotta pay for the plane, you know". Eventually he got frustrated and scrapped the whole deal.
Your results may vary, but mine were terrible. If the guy is serious about getting his license, then do both of you a favor and recommend him to the best CFI you know.