How safe is Addis Ababa?

Addis is possibly the least 'African' capital south of the Sahara (short of the capitals of South Africa and perhaps Namibia) and as such a fairly easy introduction to African city life.

For all intents and purposes, Addis is fairly, but obviously not completely, safe. The usual caveats apply, with the difference that most westerners will stand out on the streets of Addis (but less than in other African cities), meaning your friend might be a more obvious target than most.

That said, Ethiopians are generally relaxed and friendly and, in large part due to their long-standing Christian past and through that their more euro-centric (or rather Meditteranean-centric) stance, probably identify themselves more with 'the west' than they do with 'Africa', meaning that, just because of that, a westerner is more likely to be left alone in Addis than in many other African capitals.

Still, parts of the city are definitely less safe, particularly at night. But, which major city doesn't have its less savory areas?

Your friend should be careful, specifically because he'll be travelling to a continent, country and city he's not familiar with. But he probably shouldn't have to be more careful than if it would have been any other new city.

Travel light, don't flash jewellery and gadgets, be wary of loitering crowds and overly friendly strangers (wanting to show you a 'traditional show' and then charging you loads, which seems a typical Addis scam), take a (trusted) taxi at night. Pretty much the usual.

The Australian government has a travel advisory warning about it:

We advise you to reconsider your need to travel to Ethiopia, including Addis Ababa, at this time due to the threat of terrorist attack against Western interests and ongoing tensions.

and

The US Embassy in Addis Ababa issued a message to US citizens on 29 May 2014 advising that “the embassy continues to receive credible threat reports of Al-Shabaab’s intent and capability to attack Ethiopia and western interests in Ethiopia. In recent weeks there have been several incursions along the Ethiopian-Somali border. While there is no known specific information regarding the timing or location of an attack, we would like to remind US citizens to be especially vigilant in areas where large numbers of US and western citizens congregate, including restaurants, hotels, bars, places of worship, supermarkets and shopping malls.” We advise Australians in Ethiopia to heed this warning.

Among other warnings, this gives an idea of the concern over it.

Wikitravel has a section on staying safe in Addis Ababa, should you decide to go.

Finally, Virtual Tourist has a section and pictures of warnings and dangers there.

What it comes down to will be subjective, to some extent. People live there every day without problems, while others may have had a bad run. While government advisories tend to over-stipulated the level of danger, they are there for a reason, and currently the recommendation is not to visit, at least, from the Australian and US governments. However, if your friend has been travelling in other African cities and managed, he's likely to be reasonably street smart about it - and often that's what it comes down to - being safe, being smart, and avoiding situations where you put yourself at risk.

This is a subjective question -- it is difficult to give a quantitative answer ("it is 75% safe").

However, if you trust their numbers, you can check how it fares for the crime index on Numbeo.