By single trailer do you mean to hold one horse or has one axle? I assume the trailer would be a tag along (or bumper pull) model. Personally I do not trust single axle trailers (two axles have better weight distribution and stability if a flat tire, etc). Since your friend has two horses, she/he might want to take both at once sometime therefore a double trailer would be better; I also think doubles have better resale value than a single and I think a double, because it is wider, is more stable on the road than a single (goes to ratio of height to width and those are tall horses so your friend would need a taller trailer). Lastly, since the one horse has trailer issues I feel (not knowing what the issue is) that a wider trailer of sufficient height would probably be appreciated by the horse more than a narrower single one.
They look goofy but I don't think it's a bad trailer so long as it suits your needs well... that being said idk if I personally would invest in one since they tend to have a stigma attached to them therefore likely affecting the resale value. What kind of a deal is she getting on it that a used 2 horse trailer is much more expensive?
In my area where things tend to be a bit higher priced than in other parts of the country if I could find a single horse trailer it's probably going to be between 400-800$ from fair to good condition. I bought my trailer for 1200 delivered and it's in very good condition with all the parts there.
Its got the divider, quarter windows with stakes, blue tinted windows, tack area, 7'1'' +