If her daughter is over the weight limit for RFing, I'm guessing that she's just over it... which means you can install with LATCH.
LATCH INSTALL...
Put the seat in position and clip on the lower anchors. Attach top tether LOOSELY (you will tighten this later, but for now it keeps it out of the way). Pull all the slack out of the belt. Now unvelcro the side of the seat that is closest to the tail end of the latch belt. Feed the tail end of the belt from the back of the seat through to the front of the seat...
It will look like this (only that's the tail end of a seatbelt, but same difference)...
http://farm2.static.flickr.com/1421/980610430_5a2b9432f2.jpg?v=1186020305
Kneel in the seat and pull straight up and towards the inside of the carseat to get all the remaining slack out of the lower anchor belt.
Test for movement... one hand at the belt path and gentle tugging... no reefing.
If it moves more than an inch, then repeat, kneel in the seat and pull slack out. Test again. Then feed tail end back through velcro opening to the back of the seat and re-attach velcro.
Attach top tether and tighten down.
In order for a tight install... the lower anchors almost always need to come up behind the seat and into the belt path, instead of around the seat into the belt path... watch the positioning of the lower anchors.
LAP/SHOULDER BELT INSTALL...
If you are installing with seatbelt, then you can use the same trick... only you pull the excess shoulder belt through the velcro opening and pull straight up and in. Once you have the lap portion as tight as you can make it... keep hold of the shoulder belt so that the lap portion can not loosen at all, then pull remaining shoulder portion out of retractor to engage locking feature and feed all of the shoulder portion back into retractor. Test for movement... it will probably be too loose. This next part takes two people.
Hold onto the lap and shoulder portions by the buckle and release the buckle. Allow the shoulder portion to feed back into the retractor 2 - 3 clicks and then one person sit on the seat (it helps if you lean on the seat closest to the buckle) and the other person work to re-buckle the seatbelt. Test for movement... this may need to be repeated another time or two in order for it to be tight.
Finally, tighten down top tether.
In both installs, if the vehicle seat reclines... it helps to recline the vehicle seat, install seat, and then push the vehicle seat back into it's upright position. Then tighten top tether.
It can also help to lower the little recline foot on the base of the seat. Sometimes that is all it needs if you are getting front to back movement but no side to side movement.
Hope those help.
Edited to add: If the buckle stalks are long and are partially going into the belt path, you can twist them down up to 3 full 360 degree twists. OR if the entire buckle is going into the belt path, you can do that as well. You just can NOT have part of the buckle in the path and part of it out... so, all the way in or all the way out are both acceptable.