I've seen your other post and I think your situation is more complicated. One thing at a time, what are your water parameters. Naked spines indicate poor water quality.
Naked spines also indicate poor nutrition, how long have had the lion, how often and what are you feeding them. Adding any saltwater fish as food will just be another vector of adding disease.
Lions will also be susceptible to flukes. Being in the same system as the undy, whatever diagnosis the undy gets, the same goes with the lion. Follow
@Jay Hemdal recommendations.
Are the abrasions on the side scrapes or is the skin peeling. If it's scrapes then it's likely due to aggression from the undy. If it's peeling then it's more serious than that, along with the naked spines could be velvet.