A once good friend of mine had a very similar thing happen to her.
She's of the lesbian persuasion and at the time didn't live with her GF. This guy she used to go to school with would be 'taking his daily evening stroll' and would just so happen to stop outside her house to check his phone 4 times out of 5.
She eventually plucked up the courage to go outside and chat with him about it. Apparently got the same response your friend did. She decided to leave it and if it continued to happen she'd take it further.
So her GF was staying over one night and was downstairs whilst our hero was upstairs in the bedroom having just had a shower. Her GF looks out the window and notices the dude standing there (she'd naturally been kept informed as to what was happening) with his phone held up as though taking pictures... She went straight out and after having a few swift words with him, proceeded to kick him squarely in the nuts.
Apparently a neighbour called the police but he didn't press charges or provide them with any solid statement of events, which made them think he didn't want them poking around his records for whatever reason. Never once shown his face since.
There's some odd balls out there. Just tell you friend to be careful. Better yet, tell her to get someone to kick the lycra wearing swine in the goolies.