Girls can be cruel

I believe it's British slang for making out. But my guess is that in this case there is a variation. Perhaps up to but not quite penetrative sex. More likely passionate kissing and fondling. Though I also interpreted the message as the guy was trying to get with the girl, so maybe it's slang for attempting long term?

Afaik macking on is like hitting on, flirting with a girl. Same joke's around with him being listed as "free food".