There is nothing wrong with downtube shifters. They are light, shift quickly (due to minimal cabling), and make the derailleurs very easy to adjust. However, if they are friction shifters, you have to learn how to make your shifts quickly and center the derailleur on the cog, otherwise everyone around you has to put up with lots of dinging noises. I've ridden with nothing but DT friction shifters for over 30 years (aside for a brief foray into bar-ends). Never used the combined brake/shift levers, though everyone who uses them seems to like them. They are very expensive (like >$200 for a set of levers) and probably would require same-brand new derailleurs to index properly. Do you know the year of this bike? How many cogs on the cassette? If your friend isn't already used to dual-levers, why not have her learn on the DT shifters. It will force her to take a hand off the bar (which she'll need for drinking anyway) and teach her how gears really work.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72