Getting mental could be slangy, if so, the sense of of it would be "going crazy/insane/high-strung/out-of control/going berserk (said in an "out-of-control" meaning, not just crazy/insane etc...)
An example of "going mental" (out-of-control of sorts, that is, one don´t know what to do about it...sort of.)http://community.babycentre.co.uk/post/ ... ing_mental
[another case of desperate dog owner... he he]
How do I stop my two dogs from going mental and fighting with each other .....http://forums.digitalspy.co.uk/showthread.php?t=1811867
Here, her friend is getting mental on her, meaning things are getting out of hand...even if she didn´t do anything to deserve that...by her accounts.http://www.enotalone.com/forum/showthread.php?t=270210Jealous friend does not want me around her man?...
I feel sorry for her too, because she obviously is obsessed with this guy and has a major problem with insecurity and jealousy. I suspect if you took yourself out of the picture, she'd just find someone else to focus her fears and jealousy on.
If you value your friendship, i'd bring this issue out in the open and tell her she is getting mental on you with the jealousy issues as you don't want her boyfriend, never have, never will, and you're tired of her acting like this, and that if she doesn't stop, then your friendship will be over.
So, when it comes to slang, "getting mental" has a "I am surprised by that/this, it was unexpected in a way..." ring to it.
Getting crazy is, say, is a more normal and expected thing.
Using mental, you also emphasizes that it is a problem/difficulty is hard to deal with.