I'm not actually 100% sure she's not looking for a dog to constantly carry in a purse. I hope that's not the case, though.
As long as she'll take good care of the dog - toy dogs were sort of meant to be carried around
The show bred Poms tend to be the most consistently small for sure. If she'd like a bit more size, a pet bred one would be the way to go. If she's set on a puppy, she could always keep an eye on classifieds for one.
Cavalier King Charles Spaniel. Not necessarily the healthiest breed as far as I've gleaned -- although I'd bet if you kept them a decent weight, their chances are better -- but the ones I've met have probably the best temperaments overall as a breed. In my life, I've never met a nasty Cavalier.
While it's always good to keep any dog at proper weight, keeping a Cav lean and fit won't do much to prevent the health problems the breed is overwhelmingly prone to. The vast majority of Cavaliers have a heart defect, so it isn't so much a matter of if your Cav will have heart trouble but when they will. With some, it isn't an issue until they are older but about half are affected by 5 years old.
"Heart mitral valve disease (MVD) is the leading cause of death of cavalier King Charles spaniels throughout the world. MVD is a polygenetic disease which afflicts over half of all cavaliers by age 5 years and nearly all cavaliers by age 10 years, should they survive that long."
http://www.cavalierhealth.org/mitral_valve_disease.htm
The other issue is Syrinhomyelia which is also extremely widespread, about 50% of Cavs have it. It is an issue related to shape of the skull and causes neurological symptoms.
"Syringomyelia (SM) is an extremely serious condition in which fluid-filled cavities develop within the spinal cord near the brain. It is also known as "neck scratcher's disease", because one of its common signs is scratching in the air near the neck.
The back half of the cavalier King Charles spaniel’s skull typically may be too small to accommodate all of the brain’s cerebellum, which may also be too large, and so it squeezes through the foramen magnum – the hole at the back of the skull – partially blocking the flow of cerebrospinal fluid (CSF) down the spinal cord. The variable pressure created by the abnormal flow of CSF is believed to create the SM cavities – called syrinx – in the spinal cord.
SM is rare in most breeds but has become very widespread in cavalier King Charles spaniels. The number of diagnosed cases in cavaliers has increased dramatically since 2000. Researchers estimate that up to 95% of CKCSs have Chiari-like malformation (CM or CLM) – also known as caudal occipital malformation syndrome (COMS) or occipital hypoplasia (OH), the skull bone malformation present in all cases and believed to be at least part of the cause of syringomyelia – and that more than 50% of cavaliers have SM. The severity and extent of syringomyelia also appear to get worse in each succeeding generation of cavaliers. It is worldwide in scope and not limited to any country, breeding line, or kennel, and experts report that it is believed to be inherited in the cavalier."
http://cavalierhealth.org/syringomyelia.htm
Cavs are known for having wonderful dispositions but anyone considering one should be aware that they tend to be a "heartbreak breed". They have some very serious, very widespread health problems which aren't prevented by good care or by going to a good breeder.