I don't think anyone source has gotten more merits. I know I haven't. The merit source stat page says:
I believe there was 17980 generation before 1 source was removed.

My guess is that your friend just sent more merits on some days than others. Merits that you send come back in 30 days, and he/she may have sent more on certain dates than others. I would tell them to not worry about it, as it is a volunteer thing. I'm pretty sure there's quite a few sources wasting a lot of sMerits (bottom of most generous sent 213 merits).

If they wish to spend merits faster, it might be worth considering giving more. Not sure how much he gives but I've switched to giving most posts 2 merits instead of one.

Also, merit begging though PM doesn't seem like much of an issue. I've publicly stated that I am a source, and have only gotten one PM.