How can I get over this..?

If she really is his "muse" then she will always have that special something that reaches out to him, that is intangible, that no body else has. It would seem then that nobody could change that bond between them (unless he finds another muse that is!).Remember an artist does not have to be in love with their muse for them to be inspired. So it probably will never turn into something more with them. But they DO share that special bond & its hard to break, no matter how much he cares for you. It would probably bug me - so I don't know how to tell you to get over it. All I can say is try and see it in perspective and don't get anxious about it, it will just make the situation more stressful for both of you.