The solicitors are either acting as executors or acting FOR the executors - you need to find out who the executors are. If the former, I suggest you send a shot across their bows. Whilst it is correct (as dzug says) that no exec can be compelled to wind up an estate within a year, it might be worth putting a little pressure on them. If the execs are private individuals (and they have instructed the sols) you can still apply pressure.
If it is a simple estate, there is no reason why they should not now have probate and be in a position to sort things out.
Although an executor cannot be compelled to wind up an estate within a year, I would suggest writing to them as follows - get your friend's mother to write - or even write a letter and get her to sign it.
"it is now 6 months since my husband died and as far as I am aware, probate has not yet/was granted on (date). I have received no further information. IN view of the delay, I am alarmed that I have received no information from you. It is now my intention to apply for a summons from the District Probate Registry for a full inventory and account under s25 Administration of Estates Act 1925, and will hold the executors personally responsible in the costs of the claim. Please therefore let me have an update within 14 days of this letter, including a copy of the Will, a copy of the assets and liabilities and a full update as to the estate administration.
Please assure your clients (the executors)/be assured that I will take whatever steps necessary to conclude the due administration of my late husband's estate.!"
Now, in fairness, you can't really write such a letter until a year is up, but in my experience the phrase "hold you personally responsible in costs" normally works.