You're right, "ffrind" is the more common word in spoken Welsh for friend, but the most common adjective for friendly is "cyfeillgar" which comes from "cyfaill". Apparently the adjective "ffrindiol" exists, which is from "ffrind" - I looked for friendly in the dictionary and found it, but until today I had never heard of it. So, stick with ffrind and cyfeillgar for friend and friendly.
Cyfaill is a bit more formal. You'll see it a fair bit in writing, and it can mean friend or fellow. "That fellow" could in some contexts be translated with "y cyfaill hwnnw". And when you're talking about friends of a society, e.g. Friends of the [Insert name of institution here] usually the word used is Cyfeillion, not ffrindiau, which is too informal in that context.
Hmmm...I came upon this thread by chance, but having an interest in traditional Celtic culture, I now feel that I want to understand the distinction, if one exists. Are there English synonyms for "friend" which more closely approach the meaning of "ffrind" and "cyfaill"? Or, rather, is it the case that they are synonymous, and that "ffrind", having entered the Welsh language, simply supplanted "cyfaill" in spoken Welsh for only that particular nominal sense while leaving the terms derived from "cyfaill" in place? Would using "cyfaill" for "friend" in speech now be seen as incorrect, or rather as correct but just a bit antiquated? Tegs, I see you are in Ireland. Do you know of any Gaelic cognates to "cyfaill"? I don't know how much cognation there is between a Brythonic and a Goidelic language in this late age. Thanks for your time (I know I have a lot of questions, but this stuff fascinates me).
Perhaps "ffrind" came to be used for "friend" in spoken Welsh because of the double meaning of "cyfaill" (either "friend" or "fellow")?
Cyfaill is more formal. I think you might be overthinking the borrowing process - there's nothing particularly surprising about a loanword supplanting one nominal sense in common usage while derivations remain intact. Cyfaill sounds formal or literary or funny - like something someone might say giving a fancy speech or something. Using it in colloquial speech sounds funny. I'm not sure we use it to mean 'fellow' either any more (or that we use 'fellow' much in English either, to be honest).
Yes...I hear that, although I often try to buck such trends in my own common speech if I regret the loss of a particular word enough to do so. I actually find myself using "fellow" fairly often in speaking ("man" appearing a bit formal when speaking offhandedly and referring to the third person) because I prefer the word to "guy", which is what is commonly used now in the U.S., and, "dude" sounds simply ridiculous most of the time (unless, of course, one is pinching a marijuana cigarette between one's thumb and forefinger). Another example would be that I will use, though quite less often than the aforementioned, "gay" to mean "happy" or 'lighthearted", though as all are aware, that word has come to be used almost exclusively to refer otherwise in our day. Regardless, thanks, analeeh, for "nudging me back on to the rails" as pertains to "cyfaill" and "ffrind".