Re: Aikido v.s. Hapkido
I have a number of years of training in both arts. At the very basic level there is a lot of differences. The most obvious one is that HKD has all the striking capabilities as well as the joint locks and throws from Aikido. There is also a lot more and varied weapons work as well: staff, spear, sword, chain whip, cane, knife and more. Depending on your school, there will probably also be a lot of emphasis on ground work as well.
However, beyond the basic differences, there isn't that much between the two arts. They both come from the same root martial art in Daito Ryu, so you would expect that there are many similarities. All the throws and joint locks are the same. The basic principles of circular motion and efficiency, as well as entering, combat ranges (ma'ai in Aikido terminology) are all the same between the two arts. Once you get to the higher levels (3rd dan and above) there should be very little difference between the two beyond the weapons and striking. The Hap is the Korean translation of Ai, so as you can see, the two art names translate to the same meaning - harmonious/universal energy way. That should say something about how closely related they are.
As for the "hardness" of the art, that's very much driven by the school. Just as there are hard schools of Aikido and soft schools, there are the same in HKD. However, I would say that you're more likely to find schools on the harder end of the scale in HKD than Aikido. The two arts are very complimentary.
The one thing that I will caution your friend on is to check deeply into the credentials of the Hapkido school. Many schools are Tae Kwon Do plus a few joint locks thrown in in order to ride the HKD wave. Check for the lineage back to a student of Yung Sool Choi, the founder of the art (Contemporary of O'Sensei). If they don't have this, be really careful as it is likely to be a TKD-plus school.
The only other thing to comment on is the formalism. Most Aikido schools are very formal during class time. Most TKD schools are not (relatively speaking). This reflects a lot of the cultural differences between the countries they came from. Expect HKD schools to be a lot more chatty and have interactions between students far more than you'll find in Aikido schools.