For a start, yes, the diet IS important. Actually, the MOST important aspect of gaining muscle mass. She must be eating ENOUGH (i.e. ABOVE Maintenance), otherwise she will NOT gain at all, PERIOD.
Supplement-wise, if she's just starting, then the staples will be ADEQUATE:
* Good Fats
* BCAA's (although some protein powders have this in)
As for training, she won't lose weight is she starts lifting weights. In fact, if she GAINS muscle mass, then she might INcrease weight, since muscle weighs MORE than fat (even if she loses some fat), or just maintain.
Intensity is really up to the individual. I suggest that initially you select a weight that she can do for 10-12 reps but no MORE.
For a beginner I suggest starting at 2 sessions per week, as it will be sufficient (Baechle, et al., 2000): 2 x Full-Body sessions or an Upper and a Lower Body session. You can increase to 3 sessions after ~4-6 weeks, and make include an Upper, Lower, and Full-Body session.
1. Close-Grip Pull-Ups (do as many as possible and then move to assisted pull-ups to finish) 3 x 6-10
2. Flat Barbell Bench Press 3 x 10-12
3. Seated Dumbbell Shoulder Press 3 x 10-12
4. Dips (do as many as possible and then move to doing Bench Dips) 3 x 10-12
5. Incline Dumbbell Bicep Curl 3 x 10-12 (these can be taken out if desired)
6. Abs - 100 reps in TOTAL of any exercise/mix of exercises
1. Barbell Front Squat 2-3 x 10
2. Barbell Back Squat 2-3 x 10
3. Barbell Romanian Deadlifts 3 x 10-12
4. Standing Calf Raise 3 x 12-15
5. Abs - 100 reps in TOTAL of any exercise/mix of exercises
The programme should be written for 'conditioning' only, with the target of "strengthening and stabilizing those areas that give us greatest support, along with any areas that are at the highest risk for injury . . . along with any other individual weak links present" (Aaberg, 1999, p. 59). It is also the time to get your wife 'acquainted' with resistance training; and to establish good posture and technique with her lifts (Aaberg, 1999).
Multiple sets of primarily compound exercises should been used, because it will a) the greater frequency per muscle group will boost your wife's growth hormone more than doing an single sets, which will build more muscle mass and thus increase strength more effectively; b) increase neural activity; c) recruit more motor units in a given session, and therefore train your wife's body to work more effectively as a whole; and d) increase your wife's bone density VERY important for a female) and bone strength more than isolation exercises would (Heyward, 2006). Rest periods between sets should start at ~2 minutes, to allow her to recover from each set, and as your wife's endurance increases, she can decrease rests down to 60-90 seconds (Baechle, et. al., 2000).
I suggest that you look around and see what other females here do and possibly look at adapting some of their programmes, or creating your own for your wife from them, suited to her level and intensity.
Because she is a beginner pretty much ANYthing she does will work for her, and she will see results fairly quickly. her resistance/training programme/s should be changed every 4-6 weeks, though (and I don't mean drastically, but changing the order of exercises, or sets/reps, or recovery periods will be enough), just for variation and so that her body does not adapt to what it's doing and stop making progress.
And as for cardio, she doesn't need to do it, really. If she wants to for cardiovascular fitness, then 3 times per week for 30 minutes will be enough (ACSM, 2005).
And if she doesn't like what you do for her, then take her to a personal trainer for a few sessions, or get her to browse the forums for the "female perspective".