I certainly wouldn't like it. Suicide is, generally, a very poorly reasoned-out act, as demonstrated by all the unsuccessful attempts and horribly painful ways that people try to do it.
The situation is simplified somewhat, though, by the fact that both he and she are old. In modern countries and modern times, those couples who actually make it to where a natural, old-age-type death parts them typically aren't parted for long... once the spouse goes, either remaining partner typically loses the will to live and just quietly passes away shortly... perhaps a month or a year later. The fact that we could probably prevent this, via the application of certain stimulants or what have you, is not brought up because we see it as natural... if you're over 90 and your spouse has just died, well... what ARE you hanging around for? Is it necessarily a bad thing that you're ready to die now?
Obviously, self-immolation is a good deal more direct. However, this is not a rash decision made by a teenager who just can't face another final exam, this is a 90 year old woman who has lived the life she wanted, with the man she wanted, and wants to follow him rather than live out the few, grief-ladden years she has left.
Gotta go with the inaction here.