The seatbelt will loosen very easily over time if the seatbelt is not locked in some way. If I had only ELR seatbelts, I wouldn't use a restraint without a gated buckle unless the restraint had its own built-in locking clip (like the Meridian).
I have installed 2 Maxi Riders in cars with only ELR seatbelts, and even following installation procedures like kneeling in the seat to tighten the seatbelt, they were loose the moment I got out and started trying to wobble them at the belt path. I always do it this way to demonstrate WHY a gated buckle is needed. I then reinstall with gated buckle. I recently rechecked one of these installations (my sister's car) and it is still rock solid.
ETA: But get your sister to check that they're not ALR instead of ELR - both will lock in an emergency, but ALR are the ones you can pull out all the way and then they ratchet as you let them retract, and they are locked and you can't pull them out again unless you let them retract all the way back in. These seatbelts are awesome for restraint installation and I wish I had them!!!
Edited by fillesetjumeaux, 01 July 2012 - 10:48 AM.