All of the Viola genus species are good tortoise foods. So if you are talking about true wild violets, (Viola papilionacea) then that is a good tortoise food.
@echo722 - always good to check if something is OK to Feed or not, so thank you! I am encouraging a patch of wild violets to grow in my yard. I have also put African Violet plants right in Smidgen's enclosure. Appears to be a fav for climbing on!
Plants such as this can be just little tricky (not impossible) - they like light, but not too much light; they like moisture, but not too much moisture - you get the idea - I keep mine down at the cooler, drier end of the enclosure.
My best advice: if this is new to you, read up a bit on violets in general so you know for sure what your picking outside. If you chose African Violet plants for your enclosure, have multiple plants so you can "switch them out" if need be (they can take a beating!) Also, if you mist in your enclosure, don't spray water on this plant's leaves (they hate it and can develop mildew quite easily). All of my blooming plants like a drink of Fox Farm's Big Bloom a couple of times a month. To be on the safe side, I keep my plants in pots (as opposed to planting directly in the substrate). This lets me switch them out to feed, prune, etc. without Interference by Tortoise.