Friend of my ex bf sent me a friend request on FB... Should I message him explaining?

I haven't spoke to my ex or his friends for months now. I deleted them all off Facebook a while ago and one of his friends has recently sent me a friend request. One of the reasons why I don't want to accept the friend request is because I don't want to risk my ex checking up on me through his friend's Facebook. I'm wondering if I should message his friend and explain or if I should just leave it. It's been bothering me for a while now and I feel a bit guilty ignoring him because he was quite nice and supportive when me and the ex broke up. Should I explain that I'm sorry about not accepting his friend request but I prefer to keep the past in the past. Wish him the best etc. Just keep it short and sweet. Or should I just not bother and just ignore him? What is the best and most decent way to go about this?

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