It's hard to tell with that picture. Does she eat and swim well? Does she lie at the bottom of the tank or lie still at the top? Any rapid gill movement? The last two are indicators of poor health.
However, guppies and betta fish don't mix well. Guppies are rather active and they can be nippy. With female bettas you have less of a worry about the delicate fins being torn, but it does still happen. Besides, the guppy can can really stress the betta just because of their activity levels. Of course, betta fish are not the most social fish and do not frequently like tank mates apart from snails (although this also can end poorly, mostly for the snail). They are sedate fish and slow feeders while guppies are precisely the opposite. Guppies also don't need to be kept at 80 degrees. They are much more suited to 76-78, which doesn't sound like a big difference but really is.
The guppy also prefers hard water at a higher pH, and while the betta is an adaptable species, they really benefit from much softer water.