More starch - e.g. bananas, roasted sweet potato wedges, roasted white potatoes, white rice.
Banana and egg pancakes fried in coconut oil or butter with dark chocolate sauce would be a good primal way of fattening them up!
Also, more dairy, if they can tolerate it - full-fat cheese and milk.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.