The only concern is if you use scopes. Sometimes in practice you get used to a load's recoil. When it is suddenly changed without you really learning to adjust to it, it can cause the scope to teach you a nasty lesson. Especially in the excitement of hunting when shots are not off a bench and sometimes in an awkward position where you are not solid behind the rifle.

If you suddenly spring the fact that you've kicked the load up thirty grains and a different bullet, he might panic. Especially if he's board line recoil acceptance at the current time with the other load. He might start to worry about the new load instead of concentrating on the deer hunting. If he is shooting 70 grains and has an accurate load.. I would just let him use that. Enjoy the hunting together.