Your friend's dewat .25 is missing (at least) two parts:
This is an antique folding trigger 6.5mm black powder gun, and you can see the cylinder retaining pin (missing in your picture) which is hand-fitted, and of course the cylinder. The pin has a shaped part that fits between the retention mechanism / ejector rod and the frame. The ejector rod is present in your gun, and I guess if nobody is going to fire it and the action doesn't work anyway, all you need is a nickel plated cylinder in any caliber that "fits the hole".
Even parts that look like they might fit probably won't actually work, since these guns were all hand-fitted and hand-made in small batches, with all the parts carefully numbered since there was little or no possibility that a part would successfully interchange. But just to fill the hole, you could post some measurements (ie. the maximum size of the cylinder) and a picture and someone on the parts EE forum might have an old nickel plated cylinder that fits. A plastic sleeve or a bit of brass tubing could hold it in place and even allow the ejector rod to be "used". The rod in your friend's gun pokes out the empties, that's why there is the complicated / articulated bunch of screws, to guide it to the cylinder at the loading gate. The cylinder retaining pin is hollow and this rod usually has a small bend in it so it won't fall out of the hole.
I hope this helps! There might be someone with a better idea, or even an antique parts gun they are willing to sell. But mine works, even though you can't find ammo, and .25 is slightly too small for this one, so it might not even fit the space in your friend's frame, though the overall design (up to the trigger) looks the same.
These are "velodog" style revolvers, made for about 60 years from the late 1800s to WWII all over Europe in about a dozen calibers.
