I would certainly not recommend a friend visit Tajikistan. I recently spent five weeks living in Tajikistan. I rented an apartment in the capitol city Dushanbe. I took a few two or three day trips to the other major cities of the country and extensively traveled through out the incredibly remote regions of southern Tajikistan. I wish that I had a lot to say about this country, but there is not much to do.
In regards to language, a person can certainly get by with conversant Russian. Tajik (a dialect of farci) is quickly gaining popularity and only those that are educated or live in the capitol speak Russian now.
There are a few night clubs that are specifically targeted at ex-pats. Other than these clubs, there is no night life at all. The food is exceptionally bland and except for a few ex-pat restaurants, generally not very safe. The water is certainly not safe to drink out of a tap. There are several local brands of bottled water, these are also not safe. Coke is difficult to come buy, the main soda provider is RC Cola which has a plant in Tajikistan and uses local water to make the soda. This makes the soda often unsafe and will cause nausea and diarrhea.
The outer cities are much the same as Dushanbe. I did manage to find some great sites though. On a mountainous highway between Dushanbe and Kulyab, there are spectacular views of a huge water reservoir. It's really breathtaking. If I had more time, I would have certainly explored this option more thoroughly. Outside of Korgan-Tyube, there is huge archaeological dig, uncovering a mosque from the 12th century. The dig is obviously extremely primitive. Anyone is allowed to come on to the site and do digging for themselves. I took advantage of this opportunity and unburied a complete pot that I brought home with me.
I spent a month there in June and it was extremely hot. In several of the regions I visited in southern Tajikistan, temperatures reached 120 during the day. Air conditioning will be offered in the nicer hotels and in most apartments that rent to tourists.
The people are extremely friendly. I was invited into several homes and ate with several families that, despite their poverty, were more than happy to entertain a guest. Although these visits were extremely pleasant, they're very difficult to get through if you don't speak Russian or Tajik.