Your mom may have been thinking about iceberg lettuce, which can cause digestive upset, plus it has no nutritional value. Any non-iceberg/dark leafy lettuce is great. In the 7 rabbits I have or had as an adult, kale has been wildly popular with every single one of them and, price wise, it's equal to or cheaper than lettuce anyway - I recommend that to begin with.
Also, I neglected to mention that since you haven't been feeding leafy greens other than the one cabbage leaf, it's very important to
introduce the greens slowly. Start with 1/2-1 leaf of something, depending on the size (a leaf or piece no bigger than your flattened hand would be a good way to measure it). Monitor poops and if they look normal the next day, you can increase portion size over the next two days until you're feeding a full serving of the veggie. If, by the third day, there's no sign of tummy troubles then that veggie can go on the "safe" list and you can start the process over with a new veggie.
As for what Orrin added... raisins may have been the biggest culprit. While fruit says 1 tsp per 2 lbs body weight as a maximum, it's super important to remember that any dried fruit has a more concentrated sugar content. A raisin should be "measured" as though it's an entire grape even though it's like 1/3 the size. I do thoroughly agree that treats are as much to make us feel good seeing them happy, lol.
If you want to feed flowers and such, be sure they haven't been treated with any sort of pesticide or weed killer and check the safe/unsafe lists for garden plants.
Here's a good list of edible flowers and petals.
Edible flowers are actually considered "non leafy vegetables" for measurement purposes, so you can give 1 tbsp per 2 lbs body weight daily (because non-leafy veggies are usually compact like carrots, turnips, green beans, beets, etc., I feel that you can compress the petals down when measuring so you're not measuring air, too).
When feeding fruits, pellets and flowers and non-leafy veggies, always feed your rabbit portions based on what they SHOULD weigh, not what they DO weigh (so an overweight rabbit gets less than their actual weight's worth and an underweight rabbit gets more when it comes to pretty much everything but hay, which all rabbits should have an unlimited amount of).
We have a rose bush that blooms to an insane degree - I can't even seem to make a dent in it! It has small (carnation size at most) blooms. I just went out with a measuring cup and found that on our rose bush, 1/8 c (2 tbsp) = 3 flowers... I had actually guesstimated it to be about 1 tbsp per bloom, so I guess our rabbits can have more than I've been giving, hehe. For my 4 lb girls, they can apparently have 3 of our roses and Big Bunny can have 6.
It's a lot easier to use a 1/8 cup measuring cup than an actual tbsp measure to measure flower petals, btw. - feed 1/8 c per 4 lbs body weight.
The rose petals are wildly popular - they get more excited when I come in with roses than they do about most fruits. They go berserk when the petals start falling around them, it's like watching an epic match of hungry hungry hippos! I especially love that Nala loves them because she's on a diet - like Orrin, I don't want to give her sugary fruits and have found that flowers are a healthier alternative that she thinks are just as special.
