Hey,
I know Mundu has a SM version (
http://messenger.mundu.com/prd/php/smartphone.php ) that can be used on the PPC (obviously no touch screen capabilities). I personally use the AIM UK version on my Treo WX but it doesn't have visual alerts (audio only and if you have the device on mute, you never know you got hit on AIM). IM+ is liked by some... I have it on my 6700 and I am not too crazy about it. Verichat even though it isn't being sold or supported by Nokia is absolute crap now... their servers have gone to shit and it shows.
Agile costs a lot of money and still is buggy as well (the bugs were worth dealing with when it was free).
http://www.webmessenger.com/products/mimwinmobile5.htm Webmessenger is a monthly/yearly and I dont have any experience with it. Sorry about that...
Lastly, Octro
http://www.octro.com/ is a fairly decent app with nice alerts and what not. Give the demo a shot, it requires configuration but once you get the jabber client down it is decent. Dunno how it works for WM6 though...
-D
p.s. your mom.