The short answer is yes, it's always technically possible for a disabled person to access any Tokyo Metro station.
The long answer -- and this is from personal experience dealing with baby strollers in Tokyo -- is that while possible in theory, access is often seriously inconvenient in practice, eg. the station has 16 exits (not unusual in Tokyo...) and only one has a lift, or that there is no lift at all and you'll need to wait for a team of station staff to manually lug your friend up and down the stairs.
The reason is that parts of the Tokyo Metro are close to 100 years old (the Ginza Line opened in 1927) and, while much effort continues to be put into retrofitting old stations, the job is far from done. Here are illustrated examples of the kind of "barrier-free" (バリアフリー baria furii) features available in some stations:
And here is Tokyo Metro's official accessibility status page, unfortunately only in Japanese:
バリアフリー設備の設置状況 (or via Google Translate)
Click on any line to examine the situation for each of its stations. Looking at the Ginza Line, we see these entries for each station, with handy icons indicating what's available:
- ホーム⇔改札間設備 Between platform ("home") and ticket gates ("wicket" per Google)
- 改札⇔地上間設備 Between ticket gates and ground level
- 階段昇降機 Stairlift
- トイレ設備 [Accessible] Toilet facilities
- ハンドル形電動車いす利用可能 Electric wheelchair accessibility
Although the logic is a little odd, eg. Shibuya station (1st in list) seems to indicate there's no way to get to the platforms in row 1, but then notes that there are chair lifts between the platforms and the ticket gates in row 3.
It's also worth noting that Tokyo Metro is only one of multiple railway operators in Tokyo. The other two big ones are JR East, which runs the largest overground network (Yamanote Line, Narita Express etc) and Toei Subway, which runs 4 other underground metro lines; those links go to their respective "barrier-free" status pages, again only in Japanese.