OK folks Im no expert but Im gonna put my two pieces anyhow-
1.) I have a 9 m/o Male labrador, even though he is less than a year old he is already on adult formula fed 2x daily (3 cups at each feeding). The reason I switched is because of all the work we do his weight started dropping, even though I upped his feeding and cut back on work, he still did not hold his weight until a friend of mine suggested I switch to adult formula - its a higher calorie per serving food than the puppy shtuff. Now we are back to the regular routine, and he gained the weight back and has remained at the same weight since.
2.) MY dog has work-time and play-time, he knows the difference, but he enjoys it because it means he gets out of the kennel run and gets to roam a bit under my supervision. Either way he burns calories. we do 15 minute OB sessions 2x daily 1AM/1Early evening. I put him up until I either get home or I get an extended lunch break, usually this is a 20-30 work session where we work on marking. He usually goes out 65-80 yds during this time, I put him up. Then after dinner it is play time where he gets to play for 20-30mins with the kid or chase balls around the yard. OF course on weekends he gets out much more and we do extended work. So yes my dog burns calories for sure due to the excercise and work. I watch him carefully to make sure I dont over do it and end up with a EIC dog (Excercise Induced Collapse). Obviously in the summer when its hot, the schedule is changed to where we cut back on the mid-day routine or we dont do it altogether, or we do the strenous stuff in the EARLY AM.
3.) A healthy looking dog should have good muscle tone, and their should be a slight indentation at the waist. You should be able to easily feel the ribs but not see them. For most dogs they need about 20-30 mins of daily excercise. Purina has a visual weight guide to go by:
Your dog should look like dog number 5 in terms of shape.
Good luck figuring this out and coming to terms with your mum.
-Dix