You have to get into Tokyo once anyway to travel from Nikko to Hakone (Nikko is to the north of Tokyo, and Hakone is rather to the west). Are you coming back to Nikko into Asakusa (on Tobu railways), or are you coming into Tokyo station (on JR Shinkansen), or possibly to Shinjuku (on JR/Tobu combination train)? If you don't mind the bit of extra cost, the *easiest* in terms of time and train changes would be to come into Nikko using the JR/Tobu railway combo Limited Express train, because some of those trains get you into Shinjuku directly; from there you can take Odakyu Railway to Hakone. From Shinjuku, you can go using the Hakone Free Pass, and possibly book the Romance Car (limited express train) to cut down on the travel time if you think it's going to be tough on your mom.
Or, if you are using JR Shinkasen from JR Nikko to Tokyo, you can change trains at Tokyo station to go to Odawara to visit Hakone.
Please read the link that another poster on your other question posted:
http://www.japan-guide.com/forum/quereadisplay.html?0+67381