No, he wasn't justified.
This man frightened a woman so badly as he followed her down the street demanding money that she tried to call for help and he took the opportunity to mug her.
We don't know anything else about the date. We can speculate all we like about who invited whom, who chose the venue, what was agreed beforehand, but it isn't in the article and we weren't there. So we don't know. Personally I would have wanted to pay my share and I think she would have been safer if she had her own cash to at least get a bus or taxi with if she wanted or needed to get away alone.
But regardless, he had no right to follow her, demanding money she said she didn't have and then mugging her when she tried to call for help.
What might he have done to her if she didn't have a phone to steal? Taken her bag? Followed her to her house? Hit her? Raped her?
Because all of that will probably have been going through her mind as he harassed her along the road.