Older woman after my guy?

hi im 25 my partners 35.we have been together 4 years living together for 2.we are very happy and content .for about 6 months a woman at work whos 50 keeps pestering him.she texts and calls every day though he ignores them.she has asked him out but he said no he says she is a good friend like a mother figure and he doesnt want to hurt her feelings but she is starting to pester him more recently.she asks loads of questions about me though i havent met her.recently we got a anonymous letter through the door warning me away from him,my partner gave me her number and i texted her.she went running to him at work making stuff up that id supposedly done.to be honest its starting to get on my nerves i just wish hed put his foot down but hes to nice.i love him but i dont want this to carry on and on ,if shes not getting the message now when will she.