Sorry, but I am more than a little concerned here. First and foremost, you or your friend needs to get the vet out. People via the internet cannot assess a horse or its needs adequately at all. While the vet is there,I would suggest you give them a fecal to assess for worms so that the mare can perhaps be wormed with something safe, and specific to what she needs, rather than just giving her wormer and hoping it is right. Remember, as worried as you are about the feed causing colic-worms do to. Then, also while the vet is there, also ask about when she may be due, have her checked, etc. as well as the best way to get weight on her. As someone who is boarding this horse, it is also your responsibility for her to be safe and well cared for, and I know you are trying. I am guessing you are in southern Va, most likely southwest? Just a guess.....Good luck with her, to me it looks like it will be sooner rather than later......
I would consult with your vet and an equine nutritionist about a feeding regimen. It won't be an easy task to get her putting on decent weight while she has a foal growing inside taking a lot of the nutrients she needs herself right now to get back to healthy. Mares & foals can be a very tricky balance with feeding. Too much or too little can be equally as damaging. Definitely get a scale, it's too easy to be off going by scoops or qts.
I can share what I feed mine, but the circumstance is much different as they were all in good condition and fit (they are rode until they get uncomfortable) My broodmares get Purina Ultium Growth, alfalfa/tim 3x's day and all day turnout, free choice loose minerals as well as a block, free choice tim/orchard when they are in at night. When Lacey was in her last 90 days of pregnancy she was getting 9 lbs of Ultium split into 3 feedings along with 1 flake of alfalfa/tim & pasture. Now that she's got the monster on her side she gets 12 lbs Ultium in 3 meals (hay & pasture the same) the little guy eats seperate from her and he gets 6 lbs Ultium split into 3 meals.
I'd also find out what type/when she was dewormed and if she's up to date on her shots as broodmares require more in the vaccine dept.
Wait a minute... a rescue adopted her out in that condition? O.O I'm shocked.... Usually they will rehab before adopting out. I agree with getting a vet in about her diet. She's going to be nursing soon and she'll need all the nutrients she can handle.
What do yal think of the oils? Corn or Veggie Oil, heard tht adds calories. I use to give it to my older mare years ago.
Go with Veggie or Canola. Corn oil doesn't have any omega 3. Flax is another option (I give flax). I'd think adding alfalfa pellets would help with the weight although I'm not positive if it makes them "hot".