There was another thread about this subject recently. Here is the link:

Does Duct Cleaning really help?

Here is a paper written by the EPA. It is worth reading.

Should You Have the Air Ducts in Your Home Cleaned?

Ducts rarely have to be cleaned. It might be needed if there is mold growth or rodents were living inside. It is possible to get an an accumulation of dust during a major renovation. But even if that happened any dust particles should be picked up by the filter.

Be aware of the $199 duct cleaning specials. Those services don't do much other than putting a vacuum cleaner down each vent. This is something you can easily do yourself.

If she feels the need to do something,buy a free standing air cleaner. Costs less,does more and lasts a life time.

If the decision is made to clean the ducts go with an established company in your area. There are lots of fly-by-night operations that move from city to city to scam people. Consumer Reports and the BBB have information. The former says to expect to pay 400-1000 for a 2000 sq foot home.

The question you need to ask the certified cleaner is who certified them. Then go to the certifiers web site see what it takes to be certified and verify the certification. In addition to that, who "recommended" the cleaning?