Quick searching reveals these other names for Rohypnol (which is pronounced row-hip-nole, if you were wondering):
- cock smack
- ruff up
- roach 2 (R2)
- forget it
- Mexican Valium
It is kind of a long list but a few of these have more obvious origins. Roche is a common name for Hoffmann-La Roche who marketed the drug and easily accounts for the roach and roche variants. The drug itself has similar effects as valium, which would explain Mexican Valium and most of the other names are varied pronunciations of either roche or rope. The latter is "row-hip" without the "ih".
Bouncing from the row- and rope- pronunciations into a ruff- or roof- opening is understandable. If you transposed a few letters in the drug name you can find "roph" which is actually how I thought the drug was pronounced: "roe-fih-nal". I have no idea why I saw it that way, since it makes no sense at all. But there it is. Another potential mispronunciations is "roe-hif-nole" which again wouldn't actually work with the name but it sure sounds like a drug. Mushing "roe-hif" into "ruff" would be like mushing "row-hip" into "rope".
Another potential path is "ruff up" -> "ruffies" -> "roofies".
For what it is worth, some English dialects pronounce ruff and roof the same, which could account for the two variations: roofies and ruffies would sound the same as well.
So, all of that said, I wasn't able to find a history chronicling the street names of Rohypnol. Conjecture is probably the best we can do without finding a dedicated study but I don't think roofies is a drastic step from the drug name to bother looking into it further.