Bird of Paradise plant in need of help!!

Hi everyone may I please pick your brains. Just had a desperate call from youngest son who, having just bought his first property, decided to treat himself to some houseplants.
I haven't actually seen the poorly plant but it's a White Bird Of Paradise and now it's leaves are looking a tad sad.

He's eagerly waiting for me to pass on any advice that anyone kindly offers. Thank you on his behalf.

 

If he's only recently bought it and put it in the window it could be scorch.

If the other leaves all look ok it could be just an old leaf dying off.

 

Thanks @pete that's what I said , but he said that it's not kept in direct sunlight. He said there are several leaves with a darker central patch in varying shades of darkening colour, as depicted in these two pics.

 

Upsy, that is the giant plant we recently gave away to John for his farm.

 

Really!! How tall does it grow!!
Have you any other ideas what could be wrong with it Vicky?

 

Possible Red Spider Mite, moving out from the mid rib, I use a hand lens to spot these on the undersides of the leaf.

Yes I think he has bought a monster there, it will be up to the ceiling soon.
Not really bushy like the orange and blue version.

 

Upsydaisy is it free draining?, houseplants can get over watered.

 

This is Big BoP on the right ...


Here it is when it came out of it's pot ...


... and here it is at John's place beginning of March this year ...


It gets quite large ...

 

Ok thanks again Pete I'll warn him of Mites and I will look them up too. Yes he said it would grow tall.

 

The Strelitzia nicholai I saw in Tenerife last year were around 25/30ft high and a spread of 15/20 ft. I haven't got a pic of the whole plant just the flowers.
A pity he didn't choose Strelitzia reginae it is much easier to grow indoors.

 

Thanks Logan ,yes I know they can as I have plenty..
It's suppose to be free draining , but as I said I've not actually seen it myself. He's done a lot of research on how best to care for it and sounded quite knowledgeable..... but not on possible causes of disease/ ill health.

 

Gosh Vicky that is a whopper!!

 

That's ok then, just thought that the pot that i saw didn't have a saucer for it and that's why i asked, I know that you have a lot of plants.

 

Thanks @strongylodon ...I've actually just read that myself, apparently the white one needs to grow outside in order to get it to flower in about 4 years time, unlike the orange one.

 

Oh Ok thanks @Logan , no he had actually moved it from the tray to photograph it for me. He has wooded flooring that he treasures and protects!!