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.