Hueco's recommendation shoot and develop several test shots is the best approach. However, if you do not have access to a dark room, you probably do not have the chemicals to develop it.
The rule of thumb You need to stop for every decade of cold storage, so it probably needs to be shot a little slower than ISO 50. However, since you do not know the storage conditions before they are obtained from the previous owner, it may need to be shot even slower.
Since your friend already shot a part and a few frames had faint pictures, You have a reference that you can refer to suspect, Estimate how many stops your friends are displaying. You can use the exposure compensation of a digital camera to get a more accurate estimate. From there you can determine the ISO value can Job.
Hueco also explains: "Slide film has less latitude for exposure errors than neg."
One reason for this is that it should be displayed or projected directly. Under or overexposure affects the direct display. Exposure differences between a series of slides make projection difficult.
Another problem is that the slide film loses its density with increasing exposure. Once the highlights are blown, there are none. If you do not want to view directly but want to scan, you should be able to keep the highlighting. You can then bring the shadows up during or after scanning. (Why does that sound familiar …?)
You can also consider cross-processing.