Sulfuring is basically to keep mites away, the oil is to prevent breakage/dryness. It will not help to keep the mane/tail clean... the opposite in fact. The oil attracts dirt. If your friend were to show her horse she would have to wash the oil & sulphur off first.
When we got our first gypsy we did't do anything with her mane. It was gorgeous - it was a double mane and very very thick. Well, after a couple months half of it completely broke off and the rest of it thinned a lot. We still don't know what caused it but the one side has started growing back and thickening up again with the use of MTG. It's definitely a labor of love to groom them but I love it.
When we got our first gypsy we did't do anything with her mane. It was gorgeous - it was a double mane and very very thick. Well, after a couple months half of it completely broke off and the rest of it thinned a lot. We still don't know what caused it but the one side has started growing back and thickening up again with the use of MTG. It's definitely a labor of love to groom them but I love it.