being that it doesn't mean "one who makes others a widow" (that would be "widow-maker" (aka my dick))
also, is it unique in this regard, or are there any other instances of the "-er" suffix being used in this manner? or, really, as anything other than an agent suffix?
I thought "spider", but it happens that, etymologically speaking, that's from a proto-germanic agent noun meaning "one who spins", so it doesn't really count.
In Latin, -er is a suffix for male-gendered nouns. According to Wiktionary, "widower" comes from Old English, so the -er might have come from medieval Latin-speakers. I don't know where -er is an actual English suffix (for gender), or if it's only used in a few loanwords / leftovers from the past, though.
It's not diminutive, though you could say the r is used to make it masculine. It's not a suffix, either, it's an alternation, a strong augmentation of the original word form not a weak appendix. It's from Middle English widewer, and at the time it would probably have been more likely for the -er suffix to be spelled -ere. So they're unrelated.
that's all I needed, thanks folks.