Yep, keep the 642 and add a semi auto. They each have their merits. It's like having different sized screwdrivers, hammeers, etc. Different tools or different jobs.
BTW, if she's managing 3" or so at 15 to 20 yards with the 642, she's doing MUCH better than many of us! My sister is a darn good shot with a S&W .38 snub, proving that the general rule that snubbies are difficult to shoot is just that: a general rule, and not always true.
As for revolvers not jamming, they can and do fail, and I'm saying that as a certified wheel gun nut
. Overall though, I trust them more based on my own personal experiences, knowing that it doesn't take much of a loose grip and/or a less than ideal hold to cause stoppages in an otherwise reliable semi ... just my 2 cents.