What are we talking about here? The Queen is 96 for God´s sake - most human beings don´t reach such an age at all and are gone by the age of 10 or 20 years younger!
Being said "oh, if she can´t do it anymore, she should abdicate..." I find outrageous and pretty hilarious. In fact, if she can´t do anything anymore, she wouldn´t have to and there is no Regency or something needed, as long as the P o Wales and Camilla as well as the Cambridges particularly are around to take on the royal duties required!
I don't think Somebody implied the Queen should abdicate and, as we have discussed here, abdication is a process that could take several months at least (if not years) to be completed due to the need for ratification by the Commonwealth realms, see the Succession to the Crown Act 2013 as a case example.
I do agree, however, that, if the Queen is not capable of fully exercising her duties as Head of State, then she should let others do it in an
official capacity, and not informally. That means using the Counsellors of State more often, or going for a full-blown Regency, although the latter would, under British law, require three out of the Speaker of the House of Commons, the Lord Chancellor, the Chief Justice of England and Wales, and the Master of the Rolls to certify based on medical evidence that the Queen is physically or mentally incapable to discharge her duties. Personally I think a Regency would be better for the United Kingdom now, but the legal process has to be followed to the letter if the rule of law is to be preserved.
Having said that, I agree that, in the particular case in question, the Swedish Royal Court overstepped. We know other monarchs or former monarchs (Margrethe, Beatrix, etc.) have met the Queen privately on numerous occasions and such meetings were not made public by their respective Royal Households. I agree it may be considered rude and undiplomatic for the Queen to turn down a private call from her cousin, the King of Sweden, and go to Balmoral instead, but the Swedish Court commenting on an unofficial private visit and creating the expectation of a meeting was also inappropriate when the Queen, to everybody's knowledge, has been recently confirming attendance even at official events in the UK only at very last possible minute, sometimes on the eve or the day of the event itself.
It is significant to me, however, that the Diamond Jubilee celebrations for example will not apparently include a big social function with invited foreign monarchs like the Diamond Jubilee lunch at Windsor in 2012. Not only is the Queen probably not physically capable of hosting an event like that right now, but also she is probably reluctant to let the Prince of Wales deputize for her, or alternatively, receive greetings in a wheelchair or, otherwise, present herself in public and to other Heads of State in a frail state, as Tatiana Maria suggested in a different context.
