Locating one may take time. I see a lot of great suggestiong below and hope one of them will lead you to the right dongle. I had to use a dongle once a long time ago and swore to never buy something with one again.
I completely understand how your mother feels. I don't need all the new tech stuff and have everything I need now to do everything I'm interested in doing. I once read somewhere that the reason so many of us gain weight is that we have all these "energy saving" devices and have no reason to get off the couch. Having to get up off the couch to turn channels on the tv gave me a reason to get up! I'm glad you are taking such good care of you mother!
You can only get a replacement dongle at full price and I doubt Husqvarna still supplies those old ones, so I'm afraid your only hope is e-bay.
I once bought 5D instead of upgrading from 3D so I could give the 3D suite including the dongle to my daughter (who still uses it). For later upgrades the new key was placed at that same dongle.
I think I have one...but not sure where it is. Probably lost in a closet.
There is a group called Sew It's For Sale where you can put a request "In search of" and if anyone has one for sale they will contact you. I have bought and sold on this group with no problems. Good luck with your search. Hugs. https://groups.io/g/SewItsForSale
Hi and welcome. My suggestion is to check out ebay, etsy etc. My knowledge and use of Viking machines is limited. I have a brilliant machine dealer in the UK who often comes up with useful information so is there a dealer (s) in Hungary who may be able to help. I am very sorry but I dont feel that I can help much and I wish you good luck with your serch