It's *legal*, yes - in the sense that the sale of the square foot of land (such as it is) is a valid property transaction, and that British law entitles people to assume any name they like, however mad, provided it isn't obscene, blasphemous or assumed for fraudulent purposes (e.g. to steal another person's identity). In Britain you could adopt the name 'Emperor Ptang-Yang Kipperbang the Twenty-Seventh of Barataria' if you like, and it would be perfectly legal. So, yes, people who spend money at sites like highlandtitles.com can call themselves 'Laird of Glencoe' if they're silly enough to do that, and they won't be breaking any law.
What it is *not* is a legitimate title. Highlandtitles.com and similar scam merchants have created a fantasy riff on the Scottish title of 'laird', which historically means "a member of the gentry who owns a specific, substantial landed estate". Here's how they have inflated it:
1. There has never been a definitive list of lairds, or a precise definition of how much land you need to own to be one. But, roughly speaking, if you owned the whole valley of Glencoe, or at least the majority of it including the village called Glencoe, you could reasonably call yourself Laird of Glencoe. The scam merchants have exploited the vagueness of the term to suggest - which is *entirely* untrue - that anyone owning even a scrap of land in Scotland can call themselves a laird. Anyone who stops to think for even a second can see how stupid this idea is - imagine a country where everyone who owned so much as a pigsty had a posh title!
2. So they buffer the notion by suggesting that there's something so special about historic estates, that ownership of even a square foot of one of them entitles the new owner to call themself 'Laird of Glencoe'. This is also entirely untrue. There is only ever one person at a time entitled to call himself Laird of any given place - if several of you own bits of a place, none of you is laird of it.
3. They then state that, because the word 'Laird' has the same derivation as the word 'Lord', it's perfectly legit for a laird to call himself a lord. This also is entirely untrue. Yes, the two words have the same origin in Old English, 1,000 years ago - but over the centuries they came to mean two different things. (Just to put this in context: the words 'skirt' and shirt' have the same derivation in Old English, but they now mean two very different things!) Scotland has both Lairds and Lords, and they are entirely distinct from each other.
So, you know your friend best. If you think she would enjoy having a silly fantasy title, by all means buy her one. Just be kind and warn her to enjoy it in private, and not actually to go around calling herself 'Lady X of Glencoe ' unless she wants to make herself a figure of fun.