Driving home so will respond soon, but you can either try and help Shortfang get out yourself with a strength check as an action (you will take 3d6 acid trying to pull him out) or yes you can try and use a help action to give him advantage, however I should warn you of two things which Sylvia herself may not be aware of:

Firstly Shortfang has a strength Save of a whopping +0, so its highly debatable if he could get out under his own power. And because the slime would get to act before him, on its turn Shortfang will automatically take 6d6 acid Damage. That is on average a 21, more than enough to take him down. In which he would be unable to get himself out anyways. Buddy himself has been walking 30 ft away from the party as to not blow your stealth, so he’d need to move and dash in order to reach Shortfang or anyone else who might end up getting engulfed.