I was pressured into buying Apple Watch Series 3 to help monitor an ongoing health condition; I generally hate wearing watches and was able to give up them on them a couple decades ago when my employment changed. In addition to being annoyed by having a band of some sort on my wrist all the time, I have an occasional minor to moderate metal allergy. I have been wearing the Series 3 since April for 23 hours a day (it is only off my skin while it is charging), and I have two observations that might be helpful to your friend.
First, if I am able to wear it loosely, which is much of the time, the metal allergy isn’t too severe; I may have three tiny little red rings that are created by the sensors; but if I wear a tightly, which I tend to have to do while I’m working out, the allergic reaction is more severe and the itching begins to bother me after 2-3 hours until I can loosen it up and move it higher on my arm or switch to the other arm to let my skin “breathe” for awhile.
Regarding the tightness or looseness of the band, you can wear it surprisingly loose, as in sliding up and down your wrist in an obnoxiously annoying manner, and nearly twisting around backwards 360°, and the watch will still detect motion and heart rate with surprising accuracy. I guess the sensors are as much optical as electrical, as I see the green lasers flashing when worn loosely.
As for the three rings from the series 3, the images of the series 4 make it appear as though the sensors are completely different and considerably smaller, and the back is now made of glass. If I didn’t hate watches, and I wanted to lose considerable money in trading up to the Series 4, It looks to me as though there’s a good chance that any irritation or allergies from the sensors would be considerably less with the new model. I doubt the EKG feature would work well without firm contact, but I’d bet heart rate would still work fine.
Second, if your friend has allergy issues or dislikes the feel of a watch band in general, be it plastic or leather or otherwise, I cannot strongly enough recommend the Apple or Nike Sport Loop, which seem to be hypoallergenic and wash very easily, not to mention breathe very nicely and wick away moisture quickly.
That said, I learned very quickly to wash both the watch (it is of course water resistant) and the band daily with warm water and simple dish soap; rinse well, then quickly towel dry and allow to air dry while charging. This greatly reduces my metal allergy and any irritation I get from sweat collection beneath and in the band. I tried the plastic sport band included with my watch, and I hated it within an hour; it doesn’t breathe at all and was itchy and uncomfortable within minutes.