If an astronaut gets unconscious or ill I suppose the colleague will help her to the airlock. But are there any other measures that could be taken already outdoors? Do they have duct tape to seal a leak in the suit? Can any kind of first aid be provided?
None, really. If you examine the EVA Cuff Checklist (the one that is actually taken outside, shown here on Suni Williams' suit - note customized cover) you will see that all emergencies end in either TERMINATE EVA or ABORT EVA (ABORT EVA being faster) if working the procedure doesn't fix the problem. In other words, get back inside where you can actually do something about the problem.
If you get the Suit Pressure Emergency alarm:
Check the hardware gauge to confirm the reading (a redundant sensor to the one that sets off the alarm). If it shows low, check that your valves are closed and the O2 Actuator is in the proper position. Then abort the EVA. (The check mark means check something and fix it if it's wrong. If you find a valve open and closing it works, you're done.) If the hardware gauge shows the pressure is OK, ignore the alarm. But keep checking that gauge!
You can read the whole thing here.
To answer the medical aspects of this question, all Apollo spacesuits had a hypodermic injection port in the right thigh. It was protected by a flap on the outer ITMG layer (p. 14):
Access flaps constructed of a thermal-resistant cross section of materials are held closed by a system of snap fasteners and fire-resistant Velcro tape. These access flaps cover the entrance closure and the UCTA connector and biomedical injection area.
and had a self-sealing patch through the pressure garment assembly (p. 8):
A biomedical injection patch is built into the right thigh portion of the TLSA to permit a crewman to self-administer a hypodermic injection without jeopardizing the gas retention quality of the PGA.
According to Apollo by the Numbers, lidocaine, atropine, demerol, and marezine were available as injectables. Fortunately, they were never used.