I think G450X's assessment is spot on. I've been in the market for cabrios lately, and what I found is that 84-86, normal mileage, no rebuilds on engine or tranny, original clutch, top and suspension with decent cosmetics were around 16-19, 19-22 for similar condition 87-89. Cars with good to excellent cosmetics, top-ends, replaced top, clutch, shocks, etc. were 22-25. Some were of course priced higher, but I doubt were selling in this market. I've more or less taken my car off the market because selling in this market sucks. (I was getting offers of 13-15 for my car
)
Even in a good market, I don't think an SC cab is worth 28, unless it's really low miles, or otherwise has had a full engine rebuild, new clutch and the interior, top and paint are flawless. But if you can get 28, do it. As to your conscience selling to your friend far above market value, only you can answer that.
edit: based on your description which is a bit thin, I think a fair price for it would be 14-15k, assuming no issues on a PPI.