A summarized table from the RAE website shows what to use in each case:
RAE: pronombres personales átonos:

As you see les is incorrect when it is a reference to people in third person.
You can see some examples on WikiLengua:
A Jorge le vieron en el aeropuerto. [Uso leísta permitido]
A Jorge LO vieron en el aeropuerto. [Uso CORRECTO]
A María le vieron por la calle. [Uso leísta NO permitido]
A María LA vieron por la calle. [Uso CORRECTO]
A Jorge y a María les vieron en la playa. [Uso leísta NO permitido]
A Jorge y a María LOS vieron en la playa. [Uso CORRECTO]
Like I discussed on a previous answer "Leismo" is an accepted exception (I tend not to use it but sometimes I use it), and if you come to Spain a lot of people use it (widely in the center), but 'lo' as well and most of people from South America.
Leismo de cortesia is another accepted exception (another reference from WikiLengua):
También está aceptado el llamado «leísmo de cortesía», esto es, la utilización de le/les en sustitución de lo/la y los/las cuando se refiere a usted/ustedes.
¿Quiere que le lleve hasta el restaurante? [«Leísmo de cortesía»]
¿Quiere que LO/LA lleve hasta el restaurante? [Uso CORRECTO]
But here we come again with an exception...that is why most of the books, resources for non native speakers do not mention the use of 'le', Michael Thomas has decided to put it because if you go to Spain in many areas it is used.
It is mentioned on many books because it is a real fact to be known.
If I were a foreigner I would use only lo.
I end up with a paragraph from Wikipedia:
Direct-object le/les:
Generally, the unstressed third-person object pronouns in Spanish are lo, la, los, las. This is the current position of the Real Academia Española. This is a reasonable generalisation given that it is true in over ninety percent of cases in over ninety of the Spanish-speaking world. However, it is helpful to take note of the various exceptions to this general rule whereby le/les rather than lo, la, los, las are used. Note however that this use is rather modern and often found only in part of Spain whereas the use of lo, la, los, las is considered more traditional.
Update, look at the first comment:
To be honest I tought the third person in plural would be accepted only in case of "leismo de cortesia" but I run into this website www.elcastellano.org
One question says:
P: Uso correcto de la palabra saludándoles.
Answer from linguists:
R: Es adecuado en las zonas leístas cuando se usa como leísmo de cortesía: Me despido de ustedes, saludándoles cordialmente. De lo contrario, debe evitarse el leísmo de tercera persona plural: Pasó la tarde saludando a los presentes > Pasó la tarde saludándolos y no *Pasó la tarde saludándoles.
So it is better to use 'lo'.