It might be worth first perhaps getting a doc to have a look at her or a bloodtest done to check for inflammation to see if she's flaring or not?
If she's only recently got out of a bad flare situation it can take a while to get the appetite up again, her stomachs probably shrunk to a teeny tiny size and she may have also simply lost the desire for food (understandable as with Crohn's your mind can often learn to associate food with pain which can be a hard mentality to shake even once you're better) if her stomach has shrunk then perhaps eating little and often would be better, like a snack every hour or so?
When I needed to gain weight after my last flare I made sure I had lots of carbs (pasta, bread, even cakes lol) and would have meal replacement shakes as a drink in between meals (I got EnShake from the doctor - much nicer then EnSure and just like a normal milkshake really nice blended and each was a whopping 600 cals!) that is of course if she can tolerate gluten and dairy (I'm not sure if EnShakes contain dairy if made up with water).
Also maybe seeing a dietician would help? I saw one back in the day when I first flared and was underweight and being threatened with tube feedings and although I can't remember much of her specific advice now I remember she had lots to say and lots of advice at the time. Perhaps keep a food diary for her too so you can be certain of her calorie intake?