I had the Beat Solos, and all the negative comments and bias towards them are warranted. You are doing your friend a great favor by persuading him to get other headphones. The Solos really sound awful, and for the price it is ridiculous... I actually have a few pairs of the fake Solos that I bought with the intention of swapping some decent drivers into and the real Solos were not that much better than the fakes... The fakes were really inefficient though, they sounded lower at the same volume settings vs the real Solos... I have not heard of the other Beats likes Studio or Detox, nor the Tours, which I believe are the in-ears, but if your friend is upgrading from the Solos, he will probably love the COPs in comparision. It may sound like I am being critical of the COPs, but that is because I own many headphones that I can compare them against. As I mentioned, if your friend is upgrading from the Solos and doesnt have many other headphones to compare against, he will probably love them, but for around the same price range there are alot of options to consider, another one of which are the DT 770 Pros, also made by Beyerdynamic. Please keep in mind that I have not heard the DT 770s personally, but when I was researching the COPs, I read more than once that the DT 770s were better, and that users that owned both were disappointed in the COPs performance vs the DT 770. Your friend may also want to consider the DT 770, but if looks are important, the COPs IMO look better and can be customized. The COPs do not fold up though, so they will require a bag if they are going to be used for on the go as they are kind of bulky. I believe the same is also true for the DT 770s.
Did another quick comparision just now of the COPs vs the M50s and the M50s have more forward/aggressive highs and vocals, while the COPs is more laid back and more refined/well behaved. The soundstage is noticably better on the M50s. The bass on the COPs is cleaner and more controlled than on the M50s and is louder than the M50s on adjustable setting 3, and can still go up to 4. Both are fine headphones, and both are well built, but the COPs are better built. The COPs are also customizable and you can buy virtually all replacement parts for the COPs, including the headband pad which you cannot on the M50s. Also will add that the COPs has a removable cable that is prone to shorting, but the cable can also be purchased and replaced.
In the end it really is a preference issue, Even though I said I preferred the M50s, it is not like the M50s blow them out of the water, I mentioned that mainly to give you a point of reference. They are both IMO in the same tier and both are leagues above the Solo in SQ. The COPs are definitely an good option to consider, especially if you can get them for the right price. Afterall, that is why I currently have a pair ;)
EDIT: I recently put generic DT770 velours on the COPs and I can say I now prefer the COPs w/ generic velours over the M50s by a significant margin. The soundstage opened up ALOT and is much wider and more importantly, clearer compared to the M50s. The clearer soundstage improved the EQ balance making the COPs highs and especially the mids much more audible. The trade off is that you lose bass and efficency. The COPs were already hard to drive in stock form, but with the velours it takes even more power to reach the same volume. As for the bass, while you lost quantity, you in turn get quality. The bass is much cleaner now and does not bleed into the rest of the soundstage as it did with the stock pads.
Edited by 0ffbeat - 6/16/14 at 5:49am