Hueco's recommendation to shoot and develop multiple test shots is the best approach. However, if you don't have access to a dark room, you probably also don't have the necessary chemicals for development.

The rule of thumb is to drop one stop for every decade of cold storage, so it likely needs to be shot somewhat slower than ISO 50. However, since you don't know the storage conditions before being obtained by its previous owner, it may have to be shot even slower.

Since your friend already shot a roll and a few frames had faint images, you have a reference on which to base a guess. Estimate how many stops under exposed your friends images appear. You can fiddle with exposure compensation on a digital camera to help make a more accurate estimation. From there you can figure out an ISO value that may work.

Hueco also states, "Slide film has less latitude for exposure errors than neg."

One reason for this is it is intended to be viewed directly or projected. Under or over exposure affects direct viewing. Exposure differences among a set of slides complicates projection.

Another issue is slide film loses density with increased exposure. Once the highlights are blown, there's no there there. If your intention is not to view them directly, but to scan them, you should be able to underexpose to preserve the highlights. You can then bring the shadows up during or after scanning. (Why does that sound familiar... ?)

You might like to also consider cross processing.