My cousin and his girlfriend just split. She has two children, one of them is his. What rights does he have?

When he met his now-ex girlfriend, she already had a two-year-old daughter by someone else. They then went on to have another little girl, and now his ex-girlfriend has taken both of them.

Bearing in mind he brought up his girlfriend's daughter from the age of two, and the real father, as far as I can make out, is very distant, he is pretty much the older girl's father.

I was just curious, as I really care about my cousin; this whole split has taken him by surprise and he doesn't know what to do.

I'm in the UK, so I would prefer answers which cover UK law. Thanks.