wow, interesting. If it were me, next time... I'd replace the band myself. I think you're referrin to the metal silvery band that runs around the motor case? Depending on what glue he used, you should be fine. There are glues out there that are high temp - and I'm not talking about like cheap glue gun sticks or something. I've used a super high temp glue/sealant on my Rowena iron gasket replacement. Simply because my 1700 watt iron got too hot and the factory sealant failed. I used 600degree plus, automotive glue/sealant. Something like that would be a bit of overkill for our FW's. (I have a FW too), but there are others out there that are made to withstand heat. It shouldn't be too bad.
I'd imagine, that if the glue does fail over time... it will allow the band to come off without hurting the motor any.
Still, not a fun thing to deal with. It's always hard/frustrating or horrifying when you do the best you can with something only to have something or someone else come along and mess it up. I'm sorry that it happened to you, but I am impressed that the tech TOLD you about it. I think you're right... he sounds like a fairly honest person, who tried to do something and it didn't work out.