Helping without being asked, is this legal?
No, a player may not foist help upon you.
The rules state &quot;you may ask any other player to help you&quot; at which point the player may accept or deny the request. There is no &quot;involuntary&quot; help.

In addition, in your specific case, since help was not needed, even requesting help is not allowed, according to the official Munchkin FAQ:

Q. Am I allowed to ask for help if I am currently winning the fight? The rules say I can ask for help if I'm losing, but nothing about if I'm winning.
A. That's because if you're winning, you don't need help, and a true munchkin would never ask for help he doesn't need. Unless a card says otherwise (and a couple do, for their own weird reasons), you may not ask for help if you are winning a combat.

Source: Official FAQ