Does your friend's tank have a sump? If so then you can use pretty much any skimmer that will fit. There are often good deals on used and DIY skimmers on here.
If no, then your only choice is a hang-on type skimmer. For that size tank I would recommend a PM HOT-1 or a smaller Remora hang-on skimmer. The CPR Backpak is also a good choice.
If I recall, someone was selling a Remora recently in the Buy&Sell forum.
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Cheers,
Andrew B.
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