Puppy humping

[MissBeehiving]

I have a 15 week old lab cross (bitch), and a 3 year old Border Terrier who is very passive and usually submissive with other dogs (he'll roll over and show his tummy if we meet a dog he doesn't know). They get on very well and play happily together - the BT will tell pup off if she's too rough etc. But she will on occasion hump him - BT is at this but doesn't tell her off. He wouldn't usually take that from another dog and would warn them off.

I wonder whether BT is being too tolerant with her and whether if she thinks that she can get away with it with him that she can get away with that type of behaviour with other dogs. As she gets a bit older is this likely to change? Should I do anything? Am I worrying unnecessarily?