Stalker or...??

Hi, we think my girlfriends ex is stalking her on WA. Countless times a day ( and night) he is online the same time as her. Too many times to be a coincidence. She comes online, 1 minute later, he's online. She has him blocked on WA and after that she deleted him from her contacts list. Yesterday she got a new mobile phone and he was at it again. We spoke at 2.58am and he was there at 3am exactly, just for a minute. We had an enormous argument and its driving us apart as she says she doesnt contact him and I believe her but its uncanny. Is there anyone who can help me/us?