In reply to Mike Conlon:
> Mr Keswick is in danger of being accused of trying to "sandbag" you !
lol - agreed!
To the OP - I wholeheartedly agree that Middlefell Buttress is a bit of a nothing really and quite underwhelming once past the 1st pitch or so. I partnered a novice leader up it once and we were continually frustrated by people scrambling down it and getting in the way! Maybe less likely this time of year but still...
The easier routes at Raven in Langdale (right next to Middlefell Buttress) - such as Evening Wall (Severe) - are nice and would dry fast but it's not really one to take your first leading steps on maybe? A little bit serious to be able to focus on the gear and hard to sort things out if she gets stuck?
If the Mrs really needs basic leading practice, despite being obviously technically strong, then I would tend to agree that at this time of year Brown Slabs at Shepherds may be your best bet. There are a couple of good easy (V Diff) climbs there up the slabs following the obvious crack lines (there are also some more bold outings best avoided). They are normally done in one pitch with big tree belays over the top and an easy scramble/walk off.
The crack in the corner, Brown Slabs Crack, is v well protected but hard (and v polished where it counts) more like VS 5a really. The guidebook says something like "one for the aspiring VS leader" - presumably because it's very safe - - but I can think of far easier and relatively safe VS's. Also, personally, I wouldn't point a novice leader, no matter how strong technically, at the likes of Eve/Adam as they are quite serious routes - though she would be fine seconding them I am sure.
Still at Shepherds just along from Brown Slabs, the great classic V Diff of Little Chamonix would probably be more fun and memorable if she gets on well on Brown Slabs and wants another challenge. Each pitch has interest, with the highlights being getting off the block (ungradeable) on P2, the saddle belay above and the steep wall of P3 to finish. If the last two pitches seem a bit too interesting for a novice leader and in particular the middle block pitch, you could always let her lead pitch one (tree belay at the top on a huge ledge), you lead P2 and then let her decide whether to lead the final pitch from the saddle belay which is again more straightforward but great value at V Diff.
So, yes, my suggestion would be something easy at Brown Slabs, then swing leads on Little Cham (with you taking the middle pitch) and then see what takes your fancy.