Quite a few oils have been mentioned that may help, another one is grapefruit, but it's a case of experimenting with oils what suits you. Sometimes a particular oil may have just the opposite effect, I can speak from experience. Chamomile for instance, I got to help me to sleep better and it actually kept me awake, rather than relaxing I found it to be stimulating.
Another suggestion for depression, though not aromatherapy is St. Johns Wort, but do check with your doctor before using this as though it's herbal it can't be used with certain medications.

There are several essential oils that help uplift the spirit, The one I found that works the best is Bergamot. I would suggest that your mom may like drinking Earl Grey Tea which contains bergamot or use a few drops in an aromatherapy diffuser. Grapefruit is also good, however I find the flavour of grapefruit strong so add a drop of rosewood essential oil to it, tends to be calming as well as uplifting. There are several blends that can be made, using high, medium and base notes together to glean more of a positive effect. A very nice blend is bergamot (high note) clary sage (medium note) and neroli(base note). Three drops of bergamot, two of clary and one of neroli in a diffuser is very nice indeed. Hope this helps.  

Hey mate

For cases that are termed clinical, you chousl try a psy-medical doctor. However if you like to try some essential oil, you can try any citrus oils, such as lemon, bergamot, lemon. Neroli is a littel on the price side, but jasmine is one of, if not the best for depression cases.

hope this helps you, cheers

I use lavender oil when I feel depressed.
Hope this help

www.aromatherapy-69.com

thanks for this! my friend has been depressed lately, will pass this information on and hope it helps