Question about WhatsApp and friend

[hannah2c3w2]

There's a friend I've been trying to contact for ages.
Last few WhatsApp messages (sent within last few months) have all just just 1 grey tick next to them - what does this mean? Undelivered? Or just not opened at her end?
Meanwhile, she has changed her WhatsApp profile pic a few times during this period.. So I'm a bit confused. We've not fallen out or anything.
Anyone know what the single grey tick means? Surely her WhatsApp is working if she's able to change profile pic?

Undelivered. Can be a sign of being blocked - but not if you can still see their profile pic and last seen (if they've turned this on).

Two grey ticks means delivered but not read (unless they've turned off read receipts in which case you won't see blue ticks). Quite a useful feature is to press info on a message and you can see who's read it anc who hasn't.